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New Caledonian crows have the intuition to know that a stick being poked out of a plastic sheet must be caused by a person hiding behind it, scientists say.

Such reasoning was previously considered unique to humans.

The University of Auckland-based research is being published in the journal Proceedings of the National Academy of Sciences.

Researcher Alex H. Taylor said crows were placed in front of a plastic sheet from which a person poked a stick. When the crows saw someone leave the area behind the sheet, they approached a box of food near the sheet without fear.

But during the times when they did not see anyone exit from behind the sheet, they appeared to assume they might be poked with a stick if they approached the food.

This ability is one of a number discovered in crows. They also include the fashioning and use of tools to access food.

A video of the experiment shows a crow looking around to the plastic sheet 12 times if it lacked the reassurance that potential stick-pokers had left the area.

"We didn't really think the crows would be able to do this. We thought it would be a nice experiment to show the limitations of a corvid [crow] brain," Dr Taylor said. "A lot of people have studied this kind of thinking [in humans] and made strong claims that no other animal could do it."

The experiment was part of a larger project to better understand the unique intelligence of humans and how intelligence evolved.

Clever bird

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New Caledonian crows have shown:* Advanced tool use - such as making hooks out of wire.* A form of culture, with toolmaking styles varying by location.* Unusually big brains.* Nuclear families, including apparent affection for one another.* The ability to deduce hidden causality.